This work is concerned with micro-Brownian motion of polymer segments on a water surface, which is observed as the lateral diffusion of fluorescence probes attached to the polymer chains. Poly(vinyl octanal acetal)s labeled with phenanthrene (P : energy donor) and anthracene (A : energy acceptor) are spread on water, yielding a stable monolayer in which the probes are able to take statistically random distributions in the two-dimensional plane. The time-resolved fluorescence analysis of the energy transfer between the P and A probes allows us to evaluate the acceptor densities and lateral diffusion coefficients of polymer segments on the water surface. The result is obtained by fitting the fluorescence decay data with the theoretical prediction by the Monte Carlo simulation. It is found that this polymer forms a viscous monolayer, in which the lateral diffusion of polymer segments takes place with a diffusion coefficient of ca. 8 x 10(-8) cm(2) s(-1).